The present invention relates to solutions of organic semiconductors, and to the use thereof in the electronics industry.
The use of organic semiconductors as functional materials has been reality for some time or is expected in the near future in a number of different applications which can be ascribed to the electronics industry in the broadest sense. The development of organic transistors (O-TFTs, O-FETs), organic integrated circuits (O-ICs) and organic solar cells (O-SCs) is already very well advanced at the research stage, so that market introduction can be expected in the next few years. In the case of organic electroluminescent devices (OLEDs), the market introduction has already taken place, as confirmed, for example, by the car radios from Pioneer or a digital camera from Kodak with an “organic display”. A first product is also available on the market in the case of polymeric light-emitting diodes (PLEDs) in the form of a small display in a shaver from Philips N. V. The general structure of such PLEDs is shown in WO 90/13148. In spite of all advances, significant improvements are still necessary in order to make these displays a true competitor to the liquid-crystal displays (LCDs) which currently dominate the market.
In order to obtain full-colour display devices, it is necessary to develop coating methods which allow the three primary colours (red, green, blue) to be applied in a spatially separated manner. Since the polymers are generally applied from solution, printing processes are a means of choice here. Owing to the good controllability, the achievable resolution and the large variability, it is principally ink-jet printing (IJP) processes that are currently being worked on. In principle, however, other printing processes, such as, for example, offset printing, transfer printing or gravure printing processes, are also suitable. On the other hand, corresponding colour displays can also be produced by photolithographic processes; in this case, area-coating processes can then be used, as are also described below for single-colour displays. For all these possibilities, suitable polymer solutions which are on the one hand suitable for printing, but on the other hand also do not impair the properties of the PLEDs, are required. Suitable polymer solutions are also required for single-colour display devices, which can be produced, for example, by spin coating. Since it is generally known that the choice of solvents and also the solubility of the polymers in this solvent or the solvent mixture has a crucial effect on the morphology of the film formed, the electroluminescence behaviour can also be controlled thereby.
WO 02/069119 describes solutions of organic semiconductors for use in ink-jet printing processes in which solvent mixtures comprising a poor solvent having a relatively high boiling point and a good solvent having a relatively low boiling point are used. Evaporation of the more volatile solvent rapidly produces during drying a saturated solution of the material, which consequently precipitates. This prevents radial flow of the material to the edges of the pixel and facilitates relatively uniform material distribution. However, this method has the crucial disadvantage that the material does not dry uniformly, but instead a precipitate forms which results in clear inhomogeneities of the film. US 2003/0127977 shows, for example by SEM (scanning electron microscopy), that a film exhibits poor properties if the material precipitates during drying. This makes uniform and homogeneous electroluminescence from the pixel impossible. Furthermore, the irregularity of the film means that a uniform current flow is impossible. It is thought that preferred current channels thereby form, which ultimately result in short circuits and thus significantly lower stability of the device. Some experience which these experiments have recreated has furthermore shown that these inhomogeneous pixels result in a reduction in the luminous efficiency. Thus, although these solutions possibly result in improved applicational properties for ink-jet printing, such solutions cannot be widely used if these are accompanied by an impairment of the electroluminescence properties.
WO 02/072714 proposes solutions in a mixture of two (or also three) solvents, where both solvents have a boiling point of below 200° C. and one solvent has a boiling point between 140° C. and 200° C., which furthermore contain no benzylic CH2 and CH groups and have certain restrictions for the substituents on aromatic solvents. It is described as particularly favourable if the solution thickens rapidly. This is achieved by using binary or ternary solvent mixtures in which the organic semiconductor has the lowest solubility in the solvent having the highest boiling point or is very thick or gelatinous in this solvent. A rapid increase in viscosity then occurs during thickening. However, it is not always possible to find suitable solvent systems having such a composition in which the organic semiconductor thickens rapidly without precipitating in the process.
EP 1134269 describes that it is particularly advantageous for solutions of polymeric fluorescent substances which have at least one arylene-vinylene unit to have a content of poor solvent of not more than 10,000 ppm, preferably not more than 5000 ppm (determined by gas chromatography), where the lower limit is not defined. Elsewhere in the description, the preferred upper limit is even indicated as a content of poor solvent of not more than 1000 ppm, which corresponds to about 0.1% by weight (at comparable density). Poor solvents are described as being especially alcohols, in particular methanol, ethanol and isopropanol. The advantage of these solutions is stated to be a higher luminous efficiency, which is confirmed in one example (with a poly-para-phenylenevinylene derivative).